1. Field of the Invention
This invention relates to a process for producing a cracked distillate and hydrogen from a sulfur-containing heavy oil.
2. Description of the Prior Art
In recent years, processing heavy oils, in particular vacuum residual oil, has become a serious problem in petroleum refineries due to the tendency of crude oils to be heavier and the decreasing demand of power-plants, etc. for heavy oil. In addition, restrictions on the discharge of sulfur oxides and nitrogen oxides are becoming more strict year by year and the demand for low-sulfur light oil is on the rise.
Heretofore, a coking process, a partial combustion process and a combination process of coking and partial combustion have been employed industrially for processing a sulfur-containing oil. The coking process which is so-called as "EUREKA process" is described in The Japan Petroleum Institute ed., Process Handbook, "Thermal-Cracking-KUREHA", (78/2) A; the partial combustion process so-called as "Shell gasification/desulfurization process" is described in ibid., "Gasification-SIRM", (73/12) A; and the combination process of coking and partial combustion so-called as "flexicoking process" is described in ibid., "Thermal Cracking-ERE", (73/12) A, respectively. However, the coke obtained from coking contains sulfur components and heavy metals in such large amounts that it finds only limited application, whereas partial combustion involves problems in terms of the materials from which the apparatus is constructed because conbustion is conducted at temperatures as high as 1,300.degree. C. and higher and, in addition, an additional oxygen plant is required to provide the high oxygen concentrations. Further, the combination process of coking and partial combustion involves problems in terms of the materials from which the apparatus is constructed because combustion is conducted at temperatures as high as 950.degree. C. and higher.
A process has also been proposed using limestone or dolomite as a desulfurizing agent and a heat transfer medium to thereby crack heavy oils and conduct desulfurization at elevated temperatures (see Japanese Patent Publication No. 27443/76). However, the presence of alkali salts at elevated temperatures imposes the problem of selecting special materials for the apparatus.
A process has been proposed for catalytically cracking heavy oil using laterite or a laterite-containing catalyst, subjecting the resulting coke-laden catalyst to reduction processing, and contacting the catalyst with steam to produce a hydrogen-rich gas. As a result of further investigating this process, it has been discovered that:
(1) Gaseous sulfur compounds produced upon cracking a heavy sulfur-containing oil with a catalyst containing iron as a major component are fixed in the form of iron sulfide upon reacting with reduced iron which is produced when combusting the coke deposited on the catalyst with oxygen in an amount less than the amount theoretically required.
(2) When contacted with steam, reduced iron having fixed thereto iron sulfide produces hydrogen and, at the same time, iron sulfide is converted to iron oxides, with the production of hydrogen sulfide.
(3) When a high-sulfur heavy oil is catalytically cracked using a reduced catalyst, the amount of sulfur compounds in the cracked distillate decreases.
(4) The amount of hydrogen can be easily controlled by feeding an auxiliary fuel into the partial combustion zone and adjusting the feed amount.